Abstract: Under global climate change, aridity responses differ markedly across latitudinal zones, and quantifying the dependence of aridity on latitude is crucial for differentiated water-resources management. The Lancang River Basin in China spans approximately 13° of latitude and exhibits pronounced elevation gradients, providing an ideal setting for examining how long-term aridity variations respond to latitude. Using the United Nations Environment Programme (UNEP) aridity index and publicly available high-resolution gridded datasets, the basin was divided into 26 subregions at 0.5°N intervals. The spatiotemporal evolution of the aridity index at interannual and seasonal scales during 1940–2022 was analyzed, and trends in the aridity index as well as their relationships with latitude were quantified. The results show that: (1) the spring aridity index increased significantly (trend slope = 0.001677, Z = 2.38), driving an overall humidification trend across the basin; (2) the relationship between aridity and latitude exhibits pronounced spatial and seasonal heterogeneity: negative correlations occur in the southern basin (south of 24.75°N), positive correlations dominate the northern basin (north of 30.5°N), and the central basin is characterized by distinct seasonal transitions and a clear spatial discontinuity around 27.25°N; (3) the trend slope in the aridity index north of 27.25°N is significantly higher than that in the southern basin (p < 0.001). These findings reveal the latitudinal pattern of aridity-index changes in the Lancang River Basin and provide a scientific basis for formulating adaptive water-resources allocation strategies under climate-change conditions.